Corrosion fatigue is one of the potential failure modes of nuclear power materials in service. The results show that the fatigue life of materials in high temperature and high pressure water environment is significantly decreased in light water reactor (LWR). At present, the research on corrosion fatigue performance of LWR materials in high temperature and high pressure water mainly focuses on the influence factors such as temperature, dissolved oxygen (do), strain rate, strain amplitude and inclusions. However, the local stress concentration on the surface of the welded components, such as the stress concentration on the surface of the bolt, is inevitable. The notch effect caused by geometric discontinuity will affect the corrosion fatigue performance of nuclear power materials. Therefore, the notch effect must be considered when studying the influencing factors of corrosion fatigue performance of nuclear power materials.
Research status of high temperature and high pressure water notch fatigue for nuclear power materials
Since 1950s, the research on notch fatigue has never been interrupted. In this paper, the fatigue model of material is proposed by using the average stress model proposed by cohenson and other scholars Neuber proposed Neuber’s rule to predict notch fatigue life using local stress-strain method; molski and Glinka proposed equivalent strain density method to simulate notch stress-strain history. In recent years, the research on notch fatigue of materials has been gradually deepened in domestic counterparts. Liu Lijun studied the notch effect of high-strength bolts for bolted spherical joints of steel grid; Xi Wei et al. Proposed a notch fatigue life prediction method considering size effect; Feng Xianfeng et al. Studied notch fatigue strength of aluminum alloy in humid air environment. At the same time, with the popularization of computer simulation software, the combination of experimental analysis and simulation provides a new idea for the research of notch fatigue. These theories and methods promote the research of notch fatigue performance to a certain extent, and provide theoretical guidance and basis for solving the notch problem.
The typical parts and service materials of LWR nuclear power station with geometric discontinuities are listed in Table 1. It can be seen that different forms of geometric discontinuities will inevitably appear in nuclear power plant materials, which will produce stress concentration and greatly promote the initiation of fatigue cracks, and eventually lead to a significant decrease in fatigue life.
At present, due to the limitation of experimental equipment and methods, the experimental research on notch fatigue of materials is limited to room temperature and high temperature air environment. There is little research on notch fatigue performance of materials in high temperature and high pressure water environment of nuclear power plant. Only Sakaguchi et al. Studied the effect of notch effect on corrosion fatigue performance of nuclear power materials in simulated boiling water reactor (BWR) environment. The research focus is on the prediction of notch fatigue life in high temperature water environment, using finite element method (FEA) The strain rate and the environmental fatigue correction factor (Fen) of notch tip were calculated. The environmental fatigue life of notched specimen was predicted by the modified rate approximation method. The predicted environmental fatigue life of notched specimen was compared with that of smooth specimen. The accuracy and feasibility of the prediction method were analyzed. However, the above studies only focus on the quantitative analysis of notch environment fatigue life, and lack of in-depth research on the change mechanism of notch environment fatigue life and the corresponding evolution law of microstructure.
In recent years, China’s nuclear power has developed rapidly. The systematic research on the fatigue performance of domestic materials in high temperature and high pressure water will further enrich the basic data of corrosion fatigue of domestic nuclear power materials, deepen people’s understanding of the corrosion fatigue damage mechanism of materials, promote the localization of nuclear power materials, and provide important technical support for the long-term safe and stable operation of China’s nuclear power plants, and guarantee the active service The safe operation of nuclear power plant and the safety design of new power station have important theoretical and practical significance.
Characteristics of traditional notch fatigue problems
- 1 Characteristics of traditional notch fatigue problems
- 2 Characteristics of notch fatigue in water environment of 3 LWR nuclear power plant
- 3 Initiation and propagation of corrosion fatigue cracks in 4-notch specimens
- 4 Existing problems and Prospects
When studying the mechanical behavior of materials, three kinds of specimens with different geometrical states are usually introduced: smooth specimen, cracked specimen and notched specimen. For the notched specimen, when the notch root radius tends to infinity, the notched specimen is transformed into smooth specimen; when the notch root radius approaches infinity, the notched specimen is transformed into a cracked specimen. The problem between the specimen and the specimen can still be regarded as the problem of crack in the middle of the specimen.
The fatigue problems of smooth specimens and cracked specimens have been systematically studied, but the fatigue problems of notched specimens need to be further studied. In the fatigue process, compared with the smooth specimen, the existence of notch will lead to the change of the stress-strain state of the specimen: the stress state in front of the notch will change from uniaxial stress to triaxial stress, and the stress-strain level of the notch plane from the notch tip to the center of the specimen presents a gradient decline feature, and the local stress and strain at the root of the notch is significantly higher than that at the center of the specimen. The change of stress-strain state of notched specimens will affect the fatigue properties of materials, and then affect the fatigue life of notched specimens. Compared with smooth specimens, the fatigue problems of notched specimens have several characteristic factors: notch type, notch opening angle, notch depth and notch tip radius. A comprehensive analysis of these factors will be beneficial to the development of notch fatigue experimental technology and the study of notch fatigue mechanism in special environment.
Types of gaps
The results show that Notch types, such as sharp notch and blunt notch, u-notch and V-notch, affect the fracture behavior of the material. The stress intensity factor and notch sensitivity of different types of notches are different, which leads to the difference in the calculation of notch stress concentration factor. Lazzarin et al. Systematically studied the relationship between different notch types and stress intensity factors and the effect of different notch types on the fracture behavior of materials. According to different research purposes and research background, the selection of gap types is different. The V-shaped sharp notch is mainly used to solve the stress concentration problem caused by microcracks, and most of the engineering components are in the form of blunt state, so blunt notch is often used. The selection of u-notch and V-notch is not clearly defined, but u-notch can be replaced by V-notch with opening angle of 30o. Therefore, in order to systematically study the influence of notch geometry on fatigue properties of materials, V-shaped blunt notch specimens are widely used in the study of notch fatigue properties.
The Paris formula of fatigue crack growth behavior shows that the fatigue crack growth rate is related to the stress intensity factor of the specimen. According to lazzarin et al., the stress intensity factor of notched specimens is positively correlated with the notch depth, and the notch depth is included in the crack length term in the calculation of crack growth rate. Secondly, for the fatigue test of notched specimens, because the strain at the root of notch cannot be accurately obtained, some experiments are controlled by stress. When the load is constant, the increase of notch depth will inevitably lead to the increase of nominal stress amplitude of notch section, and the corresponding strain amplitude of notch tip will also change.
Gap opening angle
The results show that for V-type blunt notch, different notch opening angles correspond to different notch stress intensity factors, and eventually lead to the difference of stress-strain gradient from the tip to the center of the notch. With the increase of the notch opening angle, the stress-strain gradient from the notch tip to the specimen center decreases gradually. However, for V-shaped blunt notch, most people think that when the notch depth and notch tip radius are fixed, the influence of notch opening angle on fatigue properties is very small, which can be ignored in most cases, so that V-shaped blunt notch is equivalent to U-shaped notch. In order to eliminate the influence, the V-shaped blunt notch with constant notch opening angle can be used for notch fatigue performance research.
At present, notched specimens with given notch opening angle and depth but changing notch tip radius are widely used in notch fatigue test. In the high temperature and high pressure water environment of LWR nuclear power plant, there are many variable parameters in the experiment. If the geometric parameters of the notch are considered too much, the fatigue behavior of the notch will be too complex, and the subsequent results analysis will be lack of pertinence. Peterson diagram shows that the radius of the notch tip has a great influence on the stress concentration factor of the specimen. The smaller the radius is, the greater the stress concentration factor is. Boukharouba et al.  considered that under the same nominal stress conditions, the smaller the notch tip radius is, the greater the stress intensity factor at the notch root is, and the easier it is to promote the initiation of fatigue cracks. In the same way, Sakane et al. Carried out experiments on fatigue specimens with different stress concentration factors obtained from different notch tip radii. The results show that different stress concentration factors have great influence on fatigue crack initiation life, but have little effect on fatigue crack growth life.
Characteristics of notch fatigue in water environment of 3 LWR nuclear power plant
The characteristics of traditional notch fatigue tests have been described. Compared with the traditional high-temperature and high-pressure fatigue experiments, LWR has the characteristics of both high-temperature and high-pressure fatigue experiments.
Table 2 shows the main influencing factors of notch fatigue performance of LWR nuclear power plant in high temperature water environment and room temperature air.
The corrosion fatigue failure of notched specimens in water environment of LWR nuclear power plant involves four factors: material, notch geometry, alternating stress and environment. Material factors mainly include composition, microstructure, inclusions and surface state; notch geometry mainly refers to notch opening angle, notch depth and notch tip radius; stress factors mainly refer to stress amplitude, loading rate and waveform; and high temperature and high pressure water environment, as the unique influencing factors of LWR nuclear power plant materials corrosion fatigue, mainly refers to temperature, pressure and pH Do and dissolved hydrogen (DH) content. The interaction of the above four factors affects the initiation and propagation of corrosion fatigue crack in notched specimens, and ultimately affects the fatigue life of the material.
In the high temperature and high pressure water environment of LWR nuclear power plant, the fatigue life of nuclear power materials decreases to some extent compared with that in air, which shows the effect of environment promoting fatigue (EAF). Therefore, the notch geometry, environmental factors and their interaction are the key points in the research of notch fatigue behavior in LWR water environment. The crack initiation mechanism under high temperature and high pressure in nuclear power materials is analyzed.
Initiation and propagation of corrosion fatigue cracks in 4-notch specimens
The fatigue fracture of notched specimen is also a fracture problem of crack body, so the corrosion fatigue failure can be divided into two parts: crack initiation and crack propagation. For notched fatigue specimens, fatigue life refers to the number of load cycles experienced by the final fracture of the specimen. The stage before the crack length is about 0.1 mm is defined as the initiation stage of crack, and the stage life is defined as the fatigue crack initiation life; after that, the crack growth stage is defined as the fatigue crack growth life.
The fatigue crack initiation life of smooth specimens accounts for more than 50% of the whole fatigue life. The fatigue crack initiation life is greatly reduced by introducing notch, and the crack propagation becomes the main part of fatigue fracture. Although the relative composition of fatigue life has changed, crack initiation and propagation, as the basic components of notch fatigue fracture, need to be studied carefully in order to systematically understand the initiation and propagation behavior of notch fatigue crack and further analyze the environmental notch effect of materials.
Initiation of corrosion fatigue cracks in notched specimens
Kim crack initiation model and dislocation dipole model are commonly used to study corrosion fatigue crack initiation of notched specimens. It is considered that there is a local stress accumulation at the tip of the crack, and there is a critical crack initiation point at the crack tip. Hirose et al.  proposed a dislocation dipole model for calculating fatigue initiation life based on the corrosion fatigue test results of notched specimens of high-strength steel. It was considered that the initiation of corrosion fatigue cracks was related to hydrogen induced cracking. A large number of dislocation dipoles were generated at the notch tip due to cyclic stress, resulting in accelerated diffusion and enrichment of hydrogen atoms, and embrittlement of materials in the tip region. The relation between the stress concentration factor and the fatigue crack initiation life is also given.
In the high temperature and high pressure water environment of LWR nuclear power plant, the fatigue crack initiation life of notched specimens can be directly obtained by direct current potential drop (DCPD) and notch tip displacement measurement (NRD). The DCPD method is to apply constant DC current to both ends of the specimen and observe the potential mutation at the edge of the notch to determine the fatigue crack initiation life. This method can also be used to observe the fatigue crack propagation behavior. The NRD method calculates the crack tip opening displacement corresponding to the crack initiation according to the notch geometry, and the fatigue crack initiation life can be obtained by calibrating in the experiment.
Corrosion fatigue crack propagation of notched specimens
The results of Umeda et al. Show that the stress concentration factor of notched specimens is greater than 1 (that of smooth specimens is equal to 1). Under the conditions of different stress concentration factors, the crack initiation life of notches decreases sharply with the increase of stress concentration factors, while the crack growth life of notches only slightly changes. This shows that the notch geometry has a significant effect on the fatigue crack initiation life, and once the crack is initiated, the fatigue crack growth life is independent of the notch geometry. Therefore, the fatigue crack growth mechanism of smooth specimens can be used for reference in the analysis of notch fatigue crack growth behavior in LWR nuclear power plant water environment.
There are two recognized mechanisms of corrosion fatigue crack growth in high temperature and high pressure water, which are hydrogen induced cracking mechanism and membrane rupture / slip dissolution mechanism. Both mechanisms are related to the fracture rate, passivation rate and ion diffusion rate of the oxide film, and the mechanism of crack propagation is also different for different experimental materials and parameters. The most direct way to distinguish the two kinds of crack propagation mechanisms is crack state and fracture morphology: macro crack bending, bifurcating and bridging between cracks in hydrogen induced cracking; the fatigue fracture surface is uneven, quasi cleavage, fan-shaped River and terrace step pattern; the macro cracks generated by membrane fracture / slip dissolution mechanism show completely flat surface crack morphology and can be observed on the fracture surface To the traces of cracks being captured.
Compared with the smooth specimen, the fatigue crack growth of notched specimen still has its unique law. Traditionally, the fatigue crack growth of smooth specimens can be divided into three stages: low growth rate region, medium growth rate region (Paris region) and high growth rate region. However, for notched specimens, Sakane et al. Considered that the crack growth rate remained unchanged during the whole process of fatigue fracture, and only in the middle and later stages of crack propagation did the crack propagation behavior of notched specimens tend to be consistent with that of smooth specimens. They think that the existence of the notch leads to the stress concentration at the tip, and a large number of cracks initiate at the root of the notch and connect with each other. On the macro level, a circular crack extends from the notch tip to the center of the specimen; for smooth specimens, usually 2,3 One or more cracks initiate on the surface of the specimen and propagate in a semicircular or semi elliptical shape. One of the cracks develops into a main crack, which leads to the fatigue failure of the specimen. Therefore, the corrosion fatigue crack growth mechanism of smooth specimens can not be fully followed when studying the notch fatigue crack growth behavior in LWR nuclear power plant water environment.
Existing problems and Prospects
At present, the research on notch fatigue performance of materials is mainly concentrated in normal temperature and high temperature air environment. There are few reports on notch fatigue performance of structural materials in LWR service environment, and lack of basic fatigue data in notch environment. Therefore, it is necessary to develop notch fatigue test technology for high temperature and high pressure water environment, carry out notch fatigue test of nuclear grade low alloy steel, stainless steel, nickel base alloy in simulated nuclear power high temperature and high pressure water, and obtain the notch fatigue strength experimental data of materials. Due to the difference of stress and strain state between notched specimens and smooth specimens, the corrosion fatigue crack initiation and propagation mechanism of smooth specimens can not be directly applied to notched specimens. It is necessary to analyze the corrosion fatigue failure mechanism of notched specimens in high temperature and high pressure water based on environmental effects and notch effects, and quantitatively evaluate the effects of notch effect, environmental effect and their interaction on the notch of nuclear power materials Based on the influence of fatigue performance, the prediction model of notch fatigue life of domestic nuclear power materials is established, which provides data support for the design, development and safety evaluation of domestic nuclear power materials.
Disclaimer: the copyright of the words, pictures and video materials reproduced on this website belongs to the original author. If it involves infringement, please contact our website for deletion at the first time.